In the 1870s, he developed the theory of pragmatism: Practical outcome determines the meaning and truth of any idea. Pragmatists celebrate application and process in the transformation of knowledge... And the discovery of truth.

Peirce further explored pragmatism in his well-known essay, Fixation of Belief (1877). To him, thought was a melody and sensations were the notes. The music continues to evolve with time and no symphony is final. "Who can be sure," he said, "of what we shall not know in a few hundred years."

A pioneer of semiotics, the study of signs and symbols, the great thinker left behind over 80,000 pages of unpublished manuscripts. He observed, "The pragmatist knows that doubt is an art which has to be acquired with difficulty."